Electronic instruments, computers and other electronic devices often have a fan for cooling. Fans are often noisy and consume additional power. In addition, moving air can introduce dust into an electronics assembly. Many systems do not require continuous cooling. For example, many systems have low-power stand-by or idle modes. In addition, many systems include motors or other high power devices that may run only intermittently. Therefore, it is desirable to control a cooling fan so that it is on only if necessary. Typically, a thermostat is used to turn on a fan when an internal temperature rises above a threshold level. Thermostats and associated control circuity add cost and complexity to electronics assemblies. There is a need for a simple low-cost fan ON/OFF control system.